This invention relates to a power device circuit, for example for automotive switching applications, comprising a power semiconductor device with a short-circuit detector. The power semiconductor device may be, for example, an insulated-gate field-effect transistor (hereinafter termed "MOSFET"), an insulated-gate bipolar transistor (hereinafter termed "IGBT"), or a bipolar transistor. The detector serves for determining whether the device load is short-circuited, e.g. whether or not most of a supply-to-return voltage is directly across the power semiconductor device when, for example, a lamp or bulb operated by the device has blown into a short-circuit condition.
Two main approaches have been previously proposed for short-circuit detection, both of which involve a load voltage detector which is enabled some time, e.g 150 .mu.s, after the power semiconductor device MPWR is turned on. these two approaches are illustrated as FIGS. 7a and 7b, in which the circuit block SC' denotes the short-circuit detector.
(a) Vbl(to) detector (FIG. 7a)
In this scheme the voltage drop Vbl across the power device MPWR, between a battery supply line 1 and load terminal 11, is compared with a constant threshold voltage Vbl(to) from a reference circuit REF. In a typical 12 volt automotive system, Vbl(to) might be set to around 6 to 10 volts. If the voltage drop Vbl falls and remains below Vbl(to) then the load R.sub.L is considered normal, whereas if the voltage drop Vbl exceeds Vbl(to) then the load R.sub.L is considered to represent a short circuit. The load resistance which represents the transition between normal and short circuit is given by: EQU Rload=(Vbg-Vbl(to))/Iload
where Iload is the output current of the high-side power device MPWR and where Vbg is the battery to ground voltage between battery supply line 1 and return line 2.
There are problems with this approach:
(i) The "short circuit" load resistance is a strong function of battery voltage Vbg and power device output current Iload, whereas the end user's idea of a short circuit is more likely to be a constant value of Rload (e.g 10 mOhm). PA1 (ii) For low battery voltages, where Vbg&lt;Vbl(to), there can be no detection of "short circuit" conditions. Therefore the diagnostic feature is inoperative and other protection features such as over-temperature shutdown must be relied on. The device MPWR may continue dissipating large amounts of power for a considerable time causing an unacceptable temperature rise in the overall system or module. PA1 (iii) Simplest detectors SC of this form use avalanche diodes to determine the Vbl(to) value, but these diodes tend to have significant process variation in initial avalanche voltage and they suffer from degradation during operation. This circuit requires some physical generation of a threshold voltage Vbl(to). PA1 (i) For low battery to ground voltages, when Vbg&lt;Vlg(to), then the load is always determined as a short circuit condition, regardless of its real nature. Useful Vlg(to) values for normal operating conditions tend to preclude proper low battery voltage functionality. PA1 (ii) For higher battery voltages Vbg, very high power dissipation may occur without being detected as a short circuit if a few volts are developed across the load R.sub.L. In this case other protection features such as over-temperature shutdown must be relied on. The device MPWR may continue dissipating large amounts of power for a considerable time, causing an unacceptable temperature rise in the overall system or module. PA1 (iii) Again a physical voltage threshold is required.
(b) Vlg(to) detector (FIG. 7b)
In this scheme the load to ground line voltage Vlg, i.e. the voltage applied across the load R.sub.L, is compared with a constant threshold voltage Vlg(to). In a typical 12 volt automotive system, Vbl(to) might be set to around 2 to 6 volts. If the voltage Vlg across the load R.sub.L rises and remains above Vlg(to) then the load is considered normal whereas if the voltage Vlg across the load R.sub.L is less than Vlg(to) then the load R.sub.L is considered to be a short circuit. The load resistance which represents the transition between normal and short circuit is given by: EQU Rload=Vlg(to)/Iload
There are problems with this approach:
United States Patent specification U.S. Pat. No. 4,929,884 (our reference PHB 33363) discloses various monitor and/or protective circuits for power semiconductor devices, for example temperature-sensing circuits and a short-circuit detector circuit. The whole contents of U.S. Pat. No. 4,929,884 are hereby incorporated herein as reference material. In particular, U.S. Pat. No. 4,929,884 discloses a short-circuit detector circuit comprising (in FIG. 11 of U.S. Pat. No. 4,929,884) a potential divider of two resistances connected between the load and ground terminals. The output of the divider is fed to a current mirror and processed to provide a turn-off signal to the power semiconductor device in the event of this output being low (instead of a high voltage across the load) when the power device is on. The short-circuit detector of U.S. Pat. No. 4,929,884 is a particular form of the second approach illustrated in the present FIG. 7b. The threshold voltage Vlg(to) in this FIG. 11 circuit is physically determined by the divide ratio (RD20+RD21)/RD21 multiplied by the Vgs threshold of T117. The Vgs threshold of T117 also depends on the mirror ratio of the current mirror 402, the value of Vlow at terminal 116 and also RD23, and the threshold of the following T119-T120 inverter. Thus, the threshold voltage Vlg(to) in this FIG. 11 circuit of U.S. Pat. No. 4,929,884 is dependent on several factors which are process dependent, especially the Vgs threshold of T117 and the resistance value of RD23.
It is an aim of the present invention to provide a power semiconductor device with a short-circuit detector based on a new approach which avoids or mitigates at least most of the disadvantages of the two previous main approaches as described with reference to the present FIGS. 7a and 7b.